Parts of the Eye Here I will briefly describe various parts of the eye: Sclera The sclera is the white of the eye. "Don't shoot until you see their scleras." Exterior is smooth and white Interior is brown and grooved Extremely durable Continuous with sheath of optic nerve Tendons attached to it The Cornea The cornea is the clear bulging surface in front of the eye. It is the main refractive surface of the eye.

Primary refractive surface of the eye Index of refraction: n = 1.37 Normally transparent and uniformly thick Nearly avascular Richly supplied with nerve fibers Sensitive to foreign bodies, cold air, chemical irritation Nutrition from aqueous humor and Tears maintain oxygen exchange and water content Tears prevent scattering and improve optical quality Anterior & Posterior Chambers Index of refraction: n = 1.33 Specific viscosity of the aqueous just over 1.0 (like water, hence the name) Pressure of 15-18 mm of mercury maintains shape of eye and spacing of the elements Aqueous humor generated from blood plasma Renewal requires about an hour Glaucoma is a result of the increased fluid pressure in the eye due to the reduction or blockage of aqueous from the anterior to posterior chambers.

Iris/Pupil Sphincter muscle to constrict or dilate the pupil Pupil is the hole through which light passes Pupil diameter ranges from about 3-7 mm Area of 7-38 square mm (factor of 5) Eye color (brown, green, blue, etc.) dependent on amount and distribution of the pigment melanin Lens Transparent body enclosed in an elastic capsule Made up of proteins and water Consists of layers, like an onion, with firm nucleus, soft cortex Gradient refractive index (1.38 - 1.40) Young person can change shape of the lens via ciliary muscles Contraction of muscle causes lens to bulge At roughly age 50, the lens can no longer change shape Becomes more yellow with age: Cataracts The graph on the right shows the optical density (-log transmittance) of the lens as a function of wavelength. The curves show the change in density with age. More short wavelength light is blocked at increases ages. Vitreous Humor Retina Notice the orientation of the retina in the eye. The center of the eyeball is towards the bottom of this figure and the back of the eyeball is towards the top.

Light enters from the bottom in this figure. The light has to pass through many layers of cells before finally reaching the photoreceptors. The photoreceptors are where the light is absorbed and and transformed into the electrochemical signals used by the nervous system. This change is called TRANSDUCTION. The interior of the eyeball is the "inner" side and the exterior is the "outer" side. The nuclear layers contain cell bodies. The plexiform layers contain the connections between cells in the retina. This next picture shows a schematic of the cells in the retina: Again the light in entering from the bottom passing through all these layers before being absorbed in the receptors. You can see the two types of receptors: the rod-shaped rods and the cone-shaped cones. The signal, after transduction, is passed to the horizontal cells (H) and the bipolar cells via a layer of connections. Lateral processing takes place in this layer via the horizontal cells. The throughput is transferred to another layer of connections with the amacrine cells (A) and the ganglion cells. The amacrine cells also exhibit lateral connections in this inner plexiform layer.

The signals pass out of the eye via the ganglion cell axons which are bundled together to form the optic nerve. The retina has a similar layered structure as the gray-matter top layers of the cerebral cortex of the brain. In fact, the retina is an extension of the central nervous system (the brain and spinal cord) that forms during embryonic development. This is one reason why scientists are interested in retinal processing; the retina is an accessible part of the brain that can be easily stimulated with light. Speaking of the optic nerve... The location where the optic nerve is bundled and leaves the retina is known as the optic disk. There are no photoreceptors at the location of the optic disk and hence there is a blind spot. The scientific term for a blind spot is a scotoma. So the blind spot due to the optic disk is a natural permanent scotoma in normal vision. Here is a demonstration of the natural permanent scotoma: Close your left eye. Fixate on the cross with your right eye. This will cause the image of the cross to fall on your fovea. Adjust the viewing distance until the black spot disappears. When this happens, the image of the spot is falling on your blind spot.

What do you see (or not see) when you do this with the top figure? What happens when the gap in the bottom figure falls on your blind spot? You should see the "smiley" in the top figure disappear when it falls in your blind spot. When the gap in the bottom figure falls on the blind spot, the visual system "fills in" the line. So why don't we notice the blind spot in normal vision? For one, we have two eyes and the blind spots are in non-corresponding locations (they are nasally located (towards the nose) on the retina so the blind spots are temporal (towards the temple) in the visual field). In addition, the filling in process makes the blind spot less noticeable especially in a peripheral area of sight that has less visual acuity (the ability to see detail). As mentioned above, in front of the receptors are layers of cells through which the light must pass. In addition there is vasculature on the front surface of the retina. You can see this vasculature (or more correctly its shadow) by pressing a pen light to the side of your eyeball and gently wiggling it. What you will see looks like the figure below.

Why don't we see this regularly? As mentioned previously, the visual system is sensitive to change and when the light enters normally through the pupil, the vessels are stable. They are also small and narrow so they do not block much light however when illuminated from the side they cast a wider shadow. If you look at a deep blue field or up at the sky (not the sun) on a clear day, you may notice pulsations or squiggles moving around. These are the shadows of the red corpuscles in the blood in these vessels. The Fovea The fovea is the location on the retina of central gaze. When you look directly, or fixate, at a stimulus you the retinal locus of this central fixation is the fovea. There are only cones in the human fovea (no rods). They are thinner, elongated, any very tightly packed. Because of this, the fovea is the location of highest visual acuity and best color vision. In the diagram below you can see that the retinal layers are pulled aside (the axons of the receptors are elongated) leaving a clearer path for the light to reach the receptors.

There is actually a little indentation or pit at the location of the fovea due to this and it is a clear landmark in the retina during an ophthalmic examination. The elongated outer segments of the cones (where the photopigment is and where the transduction occurs) increase the sensitivity by increasing the amount of photopigment. There is no vasculature in the central fovea. The Macula Covering the fovea is a pigment called the macula. it is thought that the macula serves as a protective filter over the foviea that absorbs blue and ultraviolet radiation. This pigment varies from observer to oberver and is a source of individual variation in color vision. Usually we do not notice the filtering of the macula but under special conditions we can notice its presence causing what is known as Maxwell's spot. Here is a plot of the density of the macula as a function of wavelength: To see Maxwell's spot try alternately viewing through a blue and yellow filter. When looking at through the blue filter after adapting through the yellow filter you may see a dark region covering approximately the central 3° of visual angle. Try it by clicking here . No guarantees.

The middle- and long- wavelength sensitive cones are selectively adapted to the yellow so that their response is attenuated while subsequently looking through the blue, thereby enhancing the visual effect of the macula. Another demonstration of the macula is called Haidinger's Brushes. Look at a uniform blue field (again the clear sky works well for this) through a linear polarizer. You may be able to see a small yellow hourglass in the central 3° area. As you change the orientation of the polarizer, the orientation of the hour glass changes. To the right is an artists depiction of Haidinger's Brushes. The Ophthalmoscope OK, the ophthalmoscope is not a part of the eye... If you want to see into someone's eye you have a problem. Your head will block the light entering the eye. Attributed to Helmholtz, the ophthalmoscope solves this problem by shining a small beam of light in to the eye. The reflected light is then available for viewing. This is a schematic diagram showing how an ophthalmoscope works.

An alternative is to use a half silvered mirror that covers the complete entrance area and allows half the light ener the eye and then allows half of the reflecting light to pass through the mirror into the observers eye. In class, I try to borrow an ophthalmoscope so that the students can look into each other's eyes. Perhaps you can get hold of one or ask your physician or eye doctor to let you try it on him/her. One other time that one sees the inside of the eye is when you get red-eye in a photograph. What you see here is the reflection off the retina of the rhodopsin, the pink colored photopigment in the rod photoreceptors. Eye Anatomy: How Do Our Eyes Work? Eye Anatomy: How Do Our Eyes Work? Authored by Michael Garin O.D. Although it is small, the eye is a complex organ. To enable clear vision, all structures within the eye must function properly in order to capture light, focus it, and relay messages back to the brain to create a visual image. This complexity is what makes eye anatomy such a fascinating subject. When we are born our eyes are only 1.6 to 1.7 centimeters in diameter.

Over the first three years of life, the eyes grow rapidly, reaching their full size (just shy of one inch, or 2.4 cm) by the age of 13. The visible part of the eyeball makes up 1/6 of the eye’s total surface area, with the rest hidden behind the eyelids. How the Eye Works The eye is a complicated machine with many parts. It allows you not only to view objects, but to see depth, color, size, and detail. The eye works by refracting and focusing light onto the retina. When light strikes the retina, millions of rhodopsin-containing rods, which are responsible for night vision, convert the light into electrical impulses, which are sent to the brain. The brain then translates what it receives from the optic nerves so that we can understand what we see. The retina also contains millions of cones that contain iodopsin and are used for bright light vision and color perception. There are approximately 17 times more rods than cones—about 120 million rods and 7 million cones—in the retina of each eye.

Parts of the Eye The intricate anatomy of the eye allows light refraction, maintains the shape of the eye, converts light into electrical impulses, and much more. Here is a look at the various parts of the eye: The Cornea The cornea is the dome-shaped outer covering of the eye. It is like the windows of the car we discussed earlier. It protects your eye and allows you to see around you. The cornea is where light is focused. It comprises many layers, including the outer layer, the epithelium. The epithelium is often removed or cut during surgical procedures that reshape the cornea to focus light better. Unlike other organs in the human body, there are no blood vessels in the cornea, since blood vessels block light from entering the eye. Instead, the cornea receives its oxygen and nutrients from tears, from the atmosphere, and from the aqueous humor. The Sclera The Sclera is the white outer part of the eye that you can see. It provides protection and structure for the inner parts of the eye. The Conjunctiva and Lacrimal Glands The conjunctiva is a mucus layer that keeps the eye moist. It covers the sclera and the inner surfaces of the eyelids.

Infections in this area are commonly known as “ Pink Eye .” Lacrimal glands, which produce tears, are found on the outer part of each eye. The Vitreous Humor and Aqueous Humor Vitreous humor makes up approximately 80 percent of the volume of the eyeball. It is a gel-like substance in the back part of the eye that provides the shape of the eyeball. The vitreous humor is located between the lens and the retina, in an area called the vitreous cavity. Besides helping to maintain the shape of the eyeball, the vitreous cavity also provides a clear pathway for light going through the eye to the retina. The Aqueous Humor is the watery region in the front of the eyeball. It is separated into two regions, the anterior chamber in front of the iris, and the posterior chamber behind it. The canal of Schlemm drains water in this region. Blockage of this canal leads to glaucoma and other complications. The main function of the aqueous humor is to carry nutrients to the cornea and the lens and to remove waste products from inside the front of the eye via the canal of Schlemm.

The Iris and Pupil The pupil is the black hole in the center of the colored iris. It contracts when exposed to bright light and expands in darkness to allow more light into the eye. The iris is the colored part of the eye. This coloring is due to pigment cells in the tissue. People with blue eyes have less pigment in their iris than those who have brown eyes. The iris contains the sphincter pupillae, the muscle used to narrow the pupil, and the dilator pupillae, the muscle used to widen it. The iris controls how much light enters the eye by blocking extraneous light from entering the pupil. The Lens The lens is a clear structure behind the pupil that does just what a regular lens does. The main purpose of the lens is to focus light by changing shape. The ciliary body is a muscle group attached to the lens that help the lens change its shape to better focus light on the retina. As we age, our lenses naturally deteriorate, sometimes resulting in cataracts. The Retina The retina is the innermost layer of sensitive tissue that transmits light to the brain.

The retina consists of several types of cells, including a layer of rods and cones, which transform light into chemical and electrical energy that is transmitted to the optic nerves. The center of the retina contains the macula. The macula is a highly sensitive part of the retina that is responsible for our detail vision. The center of the macula, which has a major role in detail perception, is called the fovea. When there is damage to the macula, we are unable to see fine details. The Macula and Fovea The macula is the center portion of the retina. Its main function is to provide clear, distinct central vision. The fovea is the center portion of the macula that provides the sharpest vision. The fovea only contains cones. Damage to the macula or fovea often results in a decline in one’s central vision. The Optic Nerve Also known as Cranial Nerve 2, the optic nerve is what carries messages from the eye to the brain. It consists of over one million axons, which carry visual information to different parts of the brain.

The Choroid Located between the retinal pigment epithelium (see below) and the back wall of the eye, the choroid carries nutrients to the retina and the retinal pigment epithelium. The choroid is made up of melanin, which absorbs any extraneous light that may interfere with the image the eye is sending to the brain. The Retinal Pigment Epithelium The retinal pigment epithelium can be found between the retina and the choroid. The retinal pigment epithelium: Protects the retina from excess incoming light. Supplies omega 3 fatty acids for building photoreceptive membranes Supplies glucose for energy. Anatomy of the Eye When looking at the outside of the eye, several structures are readily available for viewing. The white part of the eye is a tough outer layer called the sclera. The sclera protects the inside of the eye and helps the eye keep its structure. The colored part of the eye is called the iris. This portion of the eye is what controls the amount of light that is let into the inner eye. It is actually a circular muscle that contracts or relaxes depending on the amount of light it senses.

The color of the iris is due to various pigmented cells incorporated into the iris. In the center of the iris is where we find the pupil. The pupil normally looks dark because of light absorbing pigments in the interior portions of the eye. The Eye in Cross Section When we look at a cross section of the eye, we can see the interior structures of the eye. The transparent portion at the very front of the eye is called the cornea. The cornea is a thin layer of cells that allows light to pass through the front of the eye. The lens of the eye sits behind the iris. The lens focuses the light that comes through the cornea onto the back of the eye. Once the light has traveled through the lens, it travels through the transparent vitreous. The vitreous is a water-based viscous jelly-like substance that fills the middle of the eye. It is transparent and also helps to keep the structure of the eye. The retina is a thin layer of neural cells which sits on the back of the eye. The retina is responsible for transmitting the information to the brain through the optic nerve. Human Eye Anatomy - Parts of the Eye Explained By Liz Segre ; eye illustration by Stephen Bagi Like This Page? Please Share!

The human eye has been called the most complex organ in our body. It's amazing that something so small can have so many working parts. But when you consider how difficult the task of providing vision really is, perhaps it's no wonder after all. (Mouse over the words in the eye anatomy illustration below for their definitions.) How The Eye Works In a number of ways, the human eye works much like a digital camera: Light is focused primarily by the cornea  the clear front surface of the eye, which acts like a camera lens. The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture). The eye's crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens. Light focused by the cornea and crystalline lens (and limited by the iris and pupil) then reaches the retina  the light-sensitive inner lining of the back of the eye. The retina acts like an electronic image sensor of a digital camera, converting optical images into electronic signals.

The optic nerve then transmits these signals to the visual cortex  the part of the brain that controls our sense of sight. Human Eye Anatomy (seen from above) Mouseover labels to see definitions. For more detailed information, please read the article on "Refraction and Refractive Errors: How the Eye Sees." Recommended For You Other parts of the human eye play a supporting role in the main activity of sight: Some carry fluids (such as tears and blood) to lubricate or nourish the eye. Others are muscles that allow the eye to move. Some parts protect the eye from injury (such as the lids and the epithelium of the cornea). And some are messengers, sending sensory information to the brain (such as the pain-sensing nerves in the cornea and the optic nerve behind the retina). The Look of Love Anatomy Of Attraction: The Science Behind Love At First Sight Ever wonder about the meaning behind the phrase "the look of love"? When you're gazing into your partner's eyes, pay some thought to the science behind love at first sight. The connection between eyes and love is well-documented in poem and prose since time immemorial, the stuff of whimsical fairytales and heroic legends.

But while beauty may be in the eye of the beholder, evidence increasingly suggests our brains are hardwired to both display and notice visual "clues" when it comes to a potential love interest. Sidebar continued >> One such indicator of visual attraction is larger pupils. Studies show that our pupils dilate when we feel excitement and attraction. When our gaze falls on a person or object of interest to us, the brain releases a surge of dopamine (a chemical messenger that helps control the brain's reward and pleasure centers), which triggers pupil dilation. Men perceive women with larger pupils as more romantically appealing. A 2007 study published in Hormones and Behavior showed that a woman's pupils dilate the widest during ovulation. From a biological perspective, because this characteristic is developed under the influence of estrogen, it signifies youth and therefore fertility. Research suggests that men instinctively use such indicators, among other visual cues, to assess the "value" of potential partners  as a way to seek fertile and healthy mates. Similarly, researchers at Edinburgh University found that women at their monthly fertility peak are most attracted to men with larger pupils.

According to the study published in the journal Personality and Individual Differences, larger pupils signal a man's interest, and therefore he is more indicated as a partner because interest is mutual. Conversely, the study found that when women are not at their most fertile, men's pupil size make very little difference when looking for a mate. Lingering eye contact also is a strong indicator for a potential love match. A 2009 study published in Archives of Sexual Behavior found that a man who rated a woman as attractive held eye contact for an average of 8.2 seconds, which increased his chances of "love at first sight," researchers said. If a man's gaze was around 4 seconds, the research suggests he was less than impressed. When it comes to eye color and attraction, green eyes are seen as most attractive , according to an AllAboutVision.com eye color poll of more than 70,000 people. But when it comes to matters of the heart, research suggests most people do not have an eye color preference. In fact, it appears "the look of love" is all about the pupils.

So on February 14th, look into your Valentine's eyes  chances are his or her pupils will be dilated!  A.S. [Page updated August 2016] Like This Page? Please Share! Recommended For You Gross Anatomy of the Eye by Helga Kolb – Webvision Fig. 1. View of the human eye A black-looking aperture, the pupil,that allows light to enter the eye (it appears dark because of the absorbing pigments in the retina). A colored circular muscle, the iris, which is beautifully pigmented giving us our eye’s color (the central aperture of the iris is the pupil) (Fig. 1). This circular muscle controls the size of the pupil so that more or less light, depending on conditions, is allowed to enter the eye. Eye color, or more correctly, iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is in brown eyed people and of the latter in blue and green-eyed people.

The Melanocortin-1 Receptor Gene is a regulator of eumelanin production and is located on chromosome (MCIR) 16q24.3. Point mutations in the MCIR gene will affect melanogenesis. The presence of point mutations in the MCIR gene alleles is a common feature in light skinned and blue/green eyed people (J.A.W. Metzelaar-Blok et al. 2001; P. Valverde et al. 1995). A transparent external surface, the cornea, that covers both the pupil and the iris. This is the first and most powerful lens of the optical system of the eye and allows, together with the crystalline lens the production of a sharp image at the retinal photoreceptor level. The “white of the eye”, the sclera,which forms part of the supporting wall of the eyeball. The sclera is continuous with the cornea. Furthermore this external covering of the eye is in continuity with the dura of the central nervous system. When we remove the eye from the orbit, we can see that the eye is a slightly asymmetrical sphere with an approximate sagittal diameter or length of 24 to 25 mm.

and a transverse diameter of 24 mm. It has a volume of about 6.5 cc. A cross-sectional view of the eye shows: Three different layers The external layer, formed by the sclera and cornea The intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid) The internal layer, or the sensory part of the eye, the retina Three chambers of fluid: Anterior chamber (between cornea and iris), Posterior chamber (between iris, zonule fibers and lens) and the Vitreous chamber (between the lens and the retina). The first two chambers are filled with aqueous humor whereas the vitreous chamber is filled with a more viscous fluid, the vitreous humor. The sagittal section of the eye also reveals the lens which is a transparent body located behind the iris. The lens is suspended by ligaments (called zonule fibers) attached to the anterior portion of the ciliary body. The contraction or relaxation of these ligaments as a consequence of ciliary muscle actions, changes the shape of the lens, a process called accommodation that allows us to form a sharp image on the retina.

Light rays are focused through the transparent cornea and lens upon the retina. The central point for image focus (the visual axis) in the human retina is the fovea. Here a maximally focussed image initiates resolution of the finest detail and direct transmission of that detail to the brain for the higher operations needed for perception. Slightly more nasally than the visual axis is the optic axis projecting closer to the optic nerve head. The optic axis is the longest sagittal distance between the front or vertex of the corna and the furthest posterior part of the eyeball. It is about the optic axis that the eye is rotated by the eye muscles. Some vertebrate retinas have instead of a fovea, another specialization of the central retina, known as an area centralis or a visual streak. Fig. 3. CT Horizontal transverse scan Each eyeball is held in position in the orbital cavity by various ligaments, muscles and fascial expansions that surround it (see Fig. 3). Inserted into the sclera are three pairs of muscles (6 muscles altogether).

Two pairs are rectus muscles running straight to the bony orbit of the skull orthogonal to each other (the superior rectus, the inferior rectus, the lateral rectus and the medial rectus muscles). A further pair of muscles, the oblique muscles (superior oblique and inferior oblique) are angled as the name implies obliquely. These muscles, named extraocular muscles rotate the eyeball in the orbits and allow the image to be focussed at all times on the fovea of central retina. Development of the eye. The retina is a part of the central nervous system and an ideal region of the vertebrate brain to study, because like other regions of the central nervous system, it derives from the neural tube. The retina is formed during development of the embryo from optic vesicles outpouching from two sides of the developing neural tube. The primordial optic vesicles fold back in upon themselves to form the optic cup with the inside of the cup becoming the retina and the outside remaining a single monolayer of epithelium known as the retinal pigment epithelium (Fig, 4, 3).

Initially both walls of the optic cup are one cell thick, but the cells of the inner wall divide to form a neuroepithelial layer many cells thick: the retina CLICK HERE to see a Morph of development (Quicktime movie) Sensory retinal development begins as early as the optic vesicle stage, with the migration of cell nuclei to the inner surface of the sensory retina. Additional retinal development is characterized by the formation of further layers arising from cell division and subsequent cell migration. The retina develops in an inside to outside manner: ganglion cells are formed first and photoreceptors cells become fully mature last. Further changes in retinal morphology are accomplished by simultaneous formation of multiple complex intercellular connections. Thus by 5 months of gestation (Fig. 5) most of the basic neural connections of the retina have been established (Mann, 1964). The functional synapses are made almost exclusively in the two plexiform layers and the perikarya of the nerve cells are distributed in the three nuclear layers. Photoreceptor cell maturation begins with the formation of outer segments (OS) containing visual pigment from multiple infoldings of the plasma membrane of each cell.

Outer segment formation proceeds, and the eye becomes sensitive to light at about 7 months’ gestation. Fig. 5. Schema of the layers of the developing retina around 5 months’ gestation (Modified from Odgen,1989) The final portion of the sensory retina to mature is the fovea, where the ganglion cell layer thickening begins during midgestation. The outer nuclear layer is also wider here than elsewhere in the retina and consists almost entirely of developing cone cells. The ganglion cell nuclei migrate radially outwards in a circle, leaving the fovea free of ganglion cell nuclei. Cell-cell attachments persist, however and foveal cone cells alter their shape to accomodate the movement of ganglion cells. Foveal development continues with cell rearrangements and alteration in cone shape until about 4 years after birth (Hendrickson and Yondelis, 1984; Curcio and Hendrickson, 1991). Surface membranes cover the eye cup and develop into lens, iris and cornea with the three chambers of fluid filled with aqueous and vitreous humors (Fig. 6). Fig. 6. Sagittal section of the adult human eye Summary.

In the following chapters, we will describe in greater detail the individual nerve cells that make up the retina and the functional pathways into which these neurons are organized. Eventually, we will progress to a stage where we can appreciate the summary diagrams below (Figs. 7 and 8) that show the functional wiring of two well-understood mammalian retinas, namely cat and primate retinas. References. Curcio CA, Hendrickson AE. Organization and development of the primate photoreceptor mosaic. Prog Ret Ret. 1991;10:89–120. Hendrickson AE, Youdelis C. The morphological development of the human fovea. Ophthalmology. 1984;91:603–612. [ PubMed ] Mann I. The development of the human eye. New York: Grune and Stralton; 1964. Metzelaar-Blok JAW, ter Huurne JA, Hurks HM, Keunen JE, Jager MJ, Gruis NA. (2001) Characterization of melanocortin-1 receptor gene variants in uveal melanoma patients.

Invest Ophthalmol Vis Sci. 42:1951-1954. [ PubMed ] Ogden TE. Retina: basic science and inherited retinal disease. Vol. 1. St. Louis: The CV Mosby Co.; 1989. Valverde P, Healy E, Jackson I, Rees JL, Thody AJ. (1995) Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nat Genet.;11:328-330. [ PubMed ] Colored part of the iris - crossword puzzle clue Help Clue: Colored part of the iris Colored part of the iris is a crossword puzzle clue that we have spotted 5 times. There are related clues (shown below). Referring crossword puzzle answers Likely related crossword puzzle clues Sort A-Z Eye Health: Anatomy of the Eye - VisionAware Eye Health: Anatomy of the Eye Eye Health: Anatomy of the Eye About the Eye and How It Works Diagram of the eye, viewed from the side.

National Eye Institute To understand this diagram of the eye, try to picture it as being split in two, like an apple that's been cut in half. Imagine yourself looking into the eye from the cut side. The Cornea The cornea is a transparent dome-shaped tissue that forms the front part of your eye. It functions as a window and allows light to enter your eye. It also begins the process of focusing light rays that allow you to see words and images clearly. The cornea does not contain any blood vessels, but instead contains many nerve endings that make it extremely sensitive. That is why a scratch or a loose eyelash is so painful. Jake Whalen: Living and Coping with Keratoconus Jake Whalen is a freelance copywriter who has struggled for most of his life with keratoconus . Keratoconus is a degenerative condition of the cornea , a transparent dome-shaped tissue that forms the front part of the eye. Keratoconus gradually causes the cornea to thin, bulge/protrude outward, and become cone-shaped. This creates an abnormal curvature of the eye that can cause blurred vision, glare problems, light sensitivity, and even extreme pain.

Learn more about the basics of effective eye care, including: Questions to ask when you see an eye care specialist. Check out our Getting Started Kit for more ideas to help you live well with low vision. Sign up with VisionAware to receive free weekly email alerts for more helpful information and tips for everyday living with vision loss. Aqueous Humor Aqueous humor is a clear, watery fluid contained in a chamber behind the cornea that helps bring nutrients to the eye tissue. It is made behind the lens and flows to the front of the eye, where it is drained by a tissue called the trabecular meshwork. Problems with the flow of the aqueous fluid can lead to problems with the pressure inside the eye, such as glaucoma . The Sclera The sclera is a tough white outer coating of fibrous tissue that covers your entire eyeball (all the way around) except for the cornea. The muscles that move the eye are attached to the sclera. The name sclera comes from the Greek word "skleros," which means "hard." The Iris and the Pupil The iris is a tissue inside the eye that has a hole in the center called the pupil.

The iris contains muscles that allow the pupil to become larger (open up or dilate) and smaller (close up or constrict). The iris regulates the amount of light that enters your eye by adjusting the size of the pupil opening. In bright light, the iris closes (or constricts) and makes the pupil opening smaller to restrict the amount of light that enters your eye. The iris in bright light In dim light, the iris opens (or dilates) and makes the pupil opening larger to increase the amount of light that enters your eye: The iris in dim light In addition, it is the iris that determines your eye color. People with brown eyes have heavily pigmented irises, while people with blue or lighter-colored eyes have irises with less pigment. The Lens The lens is composed of transparent, flexible tissue and is located directly behind the iris and the pupil. It is the second part of your eye, after the cornea, that helps to focus light and images on your retina. Because the lens is flexible and elastic, it can change its curved shape to focus on objects and people that are either nearby or at a distance.

The ciliary muscles, which are part of the ciliary body, are attached to the lens and contract or release to change the lens shape and curvature. The lens becomes more rounded to focus on near objects (see Figure 1): Fig. 1: A more rounded lens can focus on near objects. The lens becomes more elongated (or stretched) to focus on objects that are far away (see Figure 2): Fig. 2: A more elongated/stretched lens can focus on far objects Over time, the lens loses some of its elasticity and therefore loses some of its ability to focus on near objects. This is called presbyopia and explains why people need reading glasses as they become older. The Choroid The choroid is a dark brown membrane that is rich with blood vessels, located between the sclera and the retina. It supplies blood and nutrients to the retina and nourishes all of the other structures within the eye. The Vitreous The vitreous is the jelly-like substance that fills the inside of the back part of the eye. Over time, the vitreous becomes more liquid and can detach from the back part of the eye, which can create floaters .

If you notice new floaters or flashing lights, it is important to see your eye doctor, because a detached vitreous can cause a hole (a condition called a macular hole ) to develop in the retina. The Retina and Optic Nerve The retina is the light-sensitive tissue that lines the inside surface of the eye, much like wallpaper. Cells in the retina convert incoming light into electrical impulses. These electrical impulses are carried by the optic nerve (which resembles your television cable) to the brain, which finally interprets them as visual images. The macula is the small sensitive area in the center of the retina that provides clear central vision. The fovea is located in the center of the macula and provides the sharpest detail vision. Additional Eye Diagrams You can get a closer look at different parts of eye with this interactive eye diagram from the National Eye Institute. Healthline.com provides an interactive Human Eye in 3D online tool that can help you understand how the parts of the eye work in relation to each other.

More Information You can learn more about the parts of the eye and refractive errors, including myopia (nearsightedness), hyperopia (farsightedness), and astigmatism (both near and far blurriness) at Refractive Error and Astigmatism and A Guide to Eye Conditions .

Question: What is the name of the coloured part of an eye?
Answer: The iris